The present invention relates to treatment of phosphate-containing rock in processes which involve a grinding stage and in particular relates to improvement of the materials handling characteristics of the phosphate-containing rock.
It is well known that phosphate-containing rock (often referred to simply as "phosphate rock") can be used to produce various materials containing phosphorus. The phosphate rock which is to be treated to produce a final product is generally obtained by first mining the rock, then pre-treating it by means of a series of flotation and physical separation stages. For many processes the rock has traditionally been additionally pre-treated in a fluidised bed drier.
These pre-treatment stages may take place before the phosphate rock is transported to the processing plant, in which the treatment process to produce the final product is to take place. Alternatively one or more of the pre-treatment stages may take place at the processing plant after transportation. The pre-treated rock is then transferred, within the processing plant, to the treatment stages. In various known processes, a grinding stage is included. One such process is the production of granular triple super phosphate (GTSP), in which phosphate rock is ground and added to hot phosphoric acid.
Use of the fluid bed drier pre-treatment stage adds expense to a process and it has of late been felt to be economically desirable to avoid this stage and instead to use phosphate rock which has not been subjected to a fluid bed drying stage. Such rock is often described as "raw" (or "wet") phosphate rock.
However, it has also been found that use of raw phosphate rock in processes which comprise a grinding stage can lead to serious problems. In particular, the materials handling properties of the raw phosphate rock can be highly inconsistent. For the operation of grinding apparatus of the type commonly in place in many processing plants it is most important that the flow of the phosphate rock to the grinding apparatus is steady and consistent. If flow is inconsistent then the apparatus, which is designed to operate continuously, is forced to operate for certain periods of time in the absence of rock. This is a serious problem and leads to excessive wear on the grinding surfaces. This greatly increases the maintenance cost of the grinding apparatus. The poor properties of the raw phosphate rock which give rise to such problems also lead to significant production losses.
It is an object of the invention to improve the efficiency of processes in which raw phosphate rock is subjected to a grinding stage. In particular it is an object of the invention to improve the properties of the raw phosphate rock.